A stent for vasodilatation is known as a medical instrument which is used for the therapy of expanding a stenosed portion of a blood vessel such as a coronary artery, etc. for patients with angina, myocardial infarction, or the like. This stent for vasodilatation is, for example, made of a metallic mesh that is expanded in a cylindrical state and is inserted into a diseased part such as a stenosed blood vessel, etc. with a catheter to expand the blood vessel.
Conventionally, an austenite-based stainless steel that is a general stainless steel, such as SUS316, etc., is used for a material of this stent for vasodilatation.
On the other hand, in recent years, attention is paid to an issue of metal allergy in a human body. This metal allergy is one in which a certain kind of metal constituting a metal material is ionized according to environmental conditions for use; the formed metal ion is bound to a protein within an epidermis, a mucosal epithelium, or a cell of the human body; a chemical material which the human body does not originally possess is formed; and the living cell causes a rejection reaction against this chemical material, leading to an abnormality in the human body. As one of metal elements causing the onset of such a disease of metal allergy, Ni (nickel) is known to be a causative material.
Under such circumstances, in view of the fact that in SUS316 that is used as the above-described material of a stent for vasodilatation, which is a general austenite-based stainless steel, from 10 to 14% by mass of Ni is contained, whereas in SUS316L, from 12 to 15% by mass of Ni is contained, there is a concern of the onset of metal allergy due to elution of an Ni ion after inserting the stent for vasodilatation into the blood vessel.
As described above, from the viewpoint that the presence of Ni in the metal material is a cause of the onset of metal allergy, in the stainless steels as metal materials which have been used so far for implants, stents for vasodilatation, or instruments for living body or medical treatment, accessories or garment parts used in a state of direct contact with the skin surface of the human body, and tablewares, a stainless steel in which the elution amount of the Ni ion is decreased as far as possible is proposed as the stainless steel for avoiding the metal allergy (see PTL 1).
According to this proposal, on the assumption that it is able to cope with a lot of applications as described above, in order to realize a stainless steel in which the Ni content in the stainless steel is decreased to an extent that Ni is not substantially contained, and at the same time, not only excellent corrosion resistance, durability, strength and toughness, but also excellent formability and wear resistance are exhibited, a chemical component composition is restricted.
A basic chemical component composition of the stainless steel according to this proposal is restricted so as to contain, as a chemical component composition (% by mass), 0<C (carbon)≦0.08, 0≦Si (silicon)<0.50, 0≦Mn (manganese)≦1.50, 15≦Cr (chromium)≦30, 0≦Ni<0.05, 1≦Mo (molybdenum)≦10, 1.00<N (nitrogen)≦2.00, and 0≦Ca (calcium)<0.005, with the remainder being Fe (iron) and inevitable impurities.
In a usual austenite-based stainless steel, Fe, Cr, Ni, and Mo are essential chemical composition components, and Ni bears an important role as an austenite forming element. However, the above-described proposal is constructed in such a manner that the role of Ni in the conventional austenite-based stainless steel is complemented by other chemical composition component.
In particular, N (nitrogen) carries out an important role as a complementary component of Ni. N is an austenite forming element and is a constituent component which also contributes to an enhancement of the strength.
However, if a large amount of N is contained, the formation of Cr nitride is promoted, and the corrosion resistance is lowered by a reduction of a Cr oxide film to be caused due to a lowering of the Cr content. In addition, in particular, in the bio-environment where various ions such as a Cl (chlorine) ion, etc. are present, there are involved such problems that the corrosion resistance is conspicuously lowered and that the toughness is also lowered.
Furthermore, in the case of an austenite-based stainless steel having more than 1% by mass of N blended therein as in the above-described proposal, there is involved such a problem that the hardness becomes high due to solid-solution strengthening with N, so that the workability and moldability are deteriorated.
In view of such circumstances, the development of a stainless steel which does not substantially contain Ni, controls the content of N as far as possible, has a metal allergy onset-preventing effect, and is excellent in terms of workability and moldability and the development of a stent for vasodilatation using such a stainless steel were earnestly desired.